The present invention is related to a liquid crystal cell, more particularly to a liquid crystal cell which includes one substrate coated with a rubbed alignment layer and the other substrate coated with a photo-aligned alignment layer, and the method for fabricating that.
Generally, the liquid crystal cell comprises two substrates and liquid crystal formed between these substrates, the liquid crystal comprising anisotropic molecules. To provide an orderly alignment of liquid crystal in the cell for the uniform brightness and the high contrast ratio of the liquid crystal cell, conventional rubbing is carried out on alignment layers coating substrate. The rubbing is mechanical friction on the alignment layer so as to provide a pretilt of liquid crystal molecules defined by a pretilt angle and a pretilt angle direction. The pretilt angle refers to a polar angle and the pretilt angle direction refers to a azimuthal angle between the surface of alignment layer and the pretilt.
The pretilt of a liquid crystal molecule adjacent a first alignment layer is called a first pretilt of a first alignment layer, and the pretilt of a liquid crystal molecule adjacent a second alignment layer is called a second pretilt of a second alignment layer. Thereby, the pretilt of a liquid crystal molecule in the middle of two layers is determined by the interaction between pretilts of the first and second alignment layer.
The liquid crystal cell is classified a vertical aligned liquid crystal cell and a horizontal aligned liquid crystal cell depending upon the pretilt angle. The vertical aligned liquid crystal cell typically defines a liquid crystal cell having a pretilt angle of an alignment layer larger than 60.degree., the horizontal aligned liquid crystal cell typically refers to a liquid crystal cell having a pretilt angle of an alignment layer less than 5.degree..
There are several modes for liquid crystal cells according to relationships between a first pretilt angle direction of a first alignment layer and a second pretilt angle direction of a second alignment layer facing the first substrate. If the first pretilt angle direction is perpendicular to the second alignment direction, it is called a twisted nematic(TN) mode liquid crystal cell. If they are parallel with each other, the liquid crystal cell is called an electrically controlled birefringence(ECB) mode liquid crystal cell and a bend mode liquid crystal cell. In addition, it is called a In-Plane Switching(IPS) mode liquid crystal cell if a pretilt angle direction is shifted depending on the voltage.
A conventionally used liquid crystal display is mainly a twisted nematic liquid crystal display (TNLCD), in which the transmittance is dependent according to the viewing angle at each gray level. Especially, while the transmittance is symmetrical in the horizontal direction, the transmittance is asymmetrical in the vertical direction. Therefore, in the vertical direction, the range with inverted image phenomenon is occurred so that the vertical viewing angle becomes very limited.
To overcome said problems, a multi-domain TNLC cell such as a two-domain liquid crystal cell, and a four-domain liquid crystal cell is introduced. The multi-domain liquid crystal cell has a wider viewing angle by providing more than domains in each pixel, domains having different pretilts with respect to each other, so as to compensate for the viewing angle dependence of each domain.
The most popular process to obtain said multi-domain liquid crystal cell is a mechanical rubbing process, as shown in FIG. 1. Rubbing is performed mechanically on an entire substrate 1 coated with alignment layer 8 such as polyimide, so that microgrooves are formed on the surface of the alignment layer 8, as shown in FIG. 1a, and FIG. 1b. To divide two domains in a pixel, in FIG. 1c and FIG. 1d, a photoresist 11 is coated over entire alignment layer 8 surface, and the photoresist 11 of one domain is removed by exposing light, reverse rubbing process is carried out on one domain as shown in FIG. 1e. The remained photoresist is removed by exposing light, then, two domains are provided on the substrate 1 as shown in FIG. 1f. In the two-domain liquid crystal cell obtained thereby, the inversion of viewing angle is compensated by the aforementioned process.
However, the rubbing process causes a dust particle and/or an electrostatic discharge, so the yield is reduced and/or the substrate is damaged. The manufacturing process becomes too complicated to apply in industry, because the process includes a photolithography which is a coating of photoresist layer and removing a part of the layer by exposing light, for dividing domains.
Therefore, it is a photo-alignment method that is introduced to simplify alignment process as well as to prevent the damage of substrate. The photo-alignment is the process in which a pretilt angle direction of alignment layer is given by the irradiation of linearly polarized ultraviolet light. The alignment layer used in the photo-alignment method is mainly including PVCN(polyvinyl cinnamate). When ultraviolet light is irradiated into the photo-aligned layer coating the substrate, it causes cyclo-addition between the cinnamoyl groups of cinnamic acid side chains that belong to different photopolymers. Thereby, the direction of the photopolymer configuration i.e., the pretilt of alignment layer is aligned uniformly.
One example of the photo-alignment method is disclosured as a following process. The photo-alignment method comprises double exposure of linearly polarized ultraviolet light into a substrate coated with PVCN to determine a pretilt, the pretilt including an alignment direction, a pretilt angle direction and pretilt angle. First linearly polarized ultraviolet light is perpendicularly irradiated into the alignment layer coating substrate so as to determine a plurality of pretilt angle directions. Then, second linearly polarized light is obliquely irradiated into the alignment layer again, to determine a pretilt angle and a pretilt angle direction. The pretilt angle and pretilt angle direction are obtained by controlling the second oblique direction relative to the substrate coated with the alignment layer.
However, the photo-alignment method has problems that the process is complicated due to the double exposure and the pretilt angle is too small, for example, the obtained pretilt angles being approximately 0.15.degree., 0.26.degree. and 0.30.degree. respectively when the oblique irradiation angles are 30.degree., 45.degree. and 60.degree.. In addition, it takes long time to irradiate light into the alignment layer so total tact time is prolonged, as well, the alignment stability of photo-alignment method is weaker than that of rubbing method.